diff --git a/reikhelm-core/src/passes/cave.rs b/reikhelm-core/src/passes/cave.rs
new file mode 100644
index 0000000..7d189ec
--- /dev/null
+++ b/reikhelm-core/src/passes/cave.rs
@@ -0,0 +1,340 @@
+//! The [`CaveShaper`] shaper pass: organic cellular-automata caverns.
+//!
+//! A *shaper* that re-sculpts some already-carved rooms into natural-looking
+//! caves. It runs **after** [`RoomCarver`](crate::passes::room::RoomCarver) and
+//! **before** [`MstConnect`](crate::passes::connect::MstConnect), so connectors
+//! see the final cave footprint when they wire and carve corridors.
+//!
+//! For each qualifying room (large enough, chosen by chance) it:
+//!
+//! 1. Fills the room's bounding rect with random noise (interior cells start as
+//!    wall with probability `fill`; a one-cell border starts as wall so the cave
+//!    pulls inward), then runs `steps` rounds of the classic **4-5 cellular
+//!    automaton**: a cell becomes wall when 5+ of its 8 neighbors are wall (cells
+//!    off the local grid count as wall), which relaxes noise into blobby caverns.
+//! 2. Keeps the **largest 4-connected floor component** — so the resulting cave
+//!    is a single connected blob, never scattered pockets.
+//! 3. Re-carves the room: cells in the cave become [`Floor`](crate::map::Tile::Floor),
+//!    the rest of the room's old footprint reverts to [`Wall`](crate::map::Tile::Wall);
+//!    the region's `cells` become the cave blob and `bounds` its bounding box.
+//!
+//! If the cave would be too small (or empty), the room is left as it was. Because
+//! the cave is one connected component and [`CorridorCarver`](crate::passes::corridor::CorridorCarver)
+//! anchors to the room cell nearest the bounds center (a cave cell), a
+//! cavernized room is always reachable — connectivity is preserved.
+//!
+//! Caves stay [`Room`](crate::region::RegionKind::Room) regions: their organic
+//! outline is what reads as a cavern, so renderers need no special case.
+
+use serde::{Deserialize, Serialize};
+
+use crate::geometry::{Point, Rect};
+use crate::map::Tile;
+use crate::pass::{GenContext, Pass};
+use crate::region::RegionKind;
+use crate::rng::Rng;
+
+/// Configuration for a [`CaveShaper`] pass.
+#[derive(Clone, Copy, Debug, PartialEq, Serialize, Deserialize)]
+pub struct CaveConfig {
+    /// Probability a qualifying room is turned into a cave.
+    pub cave_chance: f64,
+    /// Minimum room extent (smaller of width/height) to qualify. Caves need room
+    /// to breathe; small rooms stay clean.
+    pub min_room: i32,
+    /// Initial wall probability for interior cells before smoothing (~0.45 gives
+    /// good caves). Clamped to `[0.0, 1.0]`.
+    pub fill: f64,
+    /// Number of cellular-automaton smoothing rounds.
+    pub steps: u32,
+}
+
+impl Default for CaveConfig {
+    /// A sensible default: about a third of larger rooms (min extent >= 7) become
+    /// caves, from 45%-wall noise smoothed four times.
+    fn default() -> Self {
+        CaveConfig {
+            cave_chance: 0.30,
+            min_room: 7,
+            fill: 0.45,
+            steps: 4,
+        }
+    }
+}
+
+/// Smallest cave (in cells) worth keeping; below this the room is left clean.
+const MIN_CAVE_CELLS: usize = 8;
+
+/// A cellular-automata cave shaper pass.
+///
+/// Construct one with [`CaveShaper::new`]; it implements [`Pass`] with the stable
+/// name `"cave_shaper"`.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub struct CaveShaper {
+    cfg: CaveConfig,
+}
+
+impl CaveShaper {
+    /// Creates a cave shaper with the given configuration.
+    pub fn new(cfg: CaveConfig) -> Self {
+        CaveShaper { cfg }
+    }
+
+    /// Builds a cave blob (as global [`Point`]s) within `bounds`, or [`None`] if
+    /// the smoothed automaton leaves nothing big enough to keep. Draws all
+    /// randomness from `rng`.
+    fn carve_cave(&self, bounds: Rect, rng: &mut Rng) -> Option<Vec<Point>> {
+        let w = bounds.w.max(0) as usize;
+        let h = bounds.h.max(0) as usize;
+        if w < 3 || h < 3 {
+            return None;
+        }
+        let idx = |x: usize, y: usize| y * w + x;
+        let fill = self.cfg.fill.clamp(0.0, 1.0);
+
+        // Initialize: border is wall, interior is wall with probability `fill`.
+        let mut wall = vec![true; w * h];
+        for y in 1..h - 1 {
+            for x in 1..w - 1 {
+                wall[idx(x, y)] = rng.chance(fill);
+            }
+        }
+
+        // Smooth with the 4-5 rule. Cells off the grid count as wall.
+        for _ in 0..self.cfg.steps {
+            let mut next = wall.clone();
+            for y in 0..h {
+                for x in 0..w {
+                    let mut walls = 0;
+                    for dy in -1i32..=1 {
+                        for dx in -1i32..=1 {
+                            if dx == 0 && dy == 0 {
+                                continue;
+                            }
+                            let nx = x as i32 + dx;
+                            let ny = y as i32 + dy;
+                            if nx < 0 || ny < 0 || nx >= w as i32 || ny >= h as i32 {
+                                walls += 1; // out of bounds counts as wall
+                            } else if wall[idx(nx as usize, ny as usize)] {
+                                walls += 1;
+                            }
+                        }
+                    }
+                    next[idx(x, y)] = walls >= 5;
+                }
+            }
+            wall = next;
+        }
+
+        // Keep the largest 4-connected floor component.
+        let component = largest_floor_component(&wall, w, h)?;
+        if component.len() < MIN_CAVE_CELLS {
+            return None;
+        }
+
+        // Map local cells to global points, in row-major order.
+        let mut cells: Vec<Point> = component
+            .into_iter()
+            .map(|i| Point::new(bounds.x + (i % w) as i32, bounds.y + (i / w) as i32))
+            .collect();
+        cells.sort_by_key(|p| (p.y, p.x));
+        Some(cells)
+    }
+}
+
+impl Pass for CaveShaper {
+    fn name(&self) -> &str {
+        "cave_shaper"
+    }
+
+    fn apply(&self, ctx: &mut GenContext, rng: &mut Rng) {
+        for idx in 0..ctx.regions.len() {
+            let region = &ctx.regions[idx];
+            if region.kind != RegionKind::Room || region.cells.is_empty() {
+                continue;
+            }
+            let bounds = region.bounds;
+            if bounds.w.min(bounds.h) < self.cfg.min_room {
+                continue;
+            }
+            if !rng.chance(self.cfg.cave_chance) {
+                continue;
+            }
+
+            let Some(cave) = self.carve_cave(bounds, rng) else {
+                continue;
+            };
+
+            // Re-carve: clear the room's old footprint, then carve the cave. The
+            // old cells are this room's only carved area at this stage (corridors
+            // run later), so clearing them touches nothing else.
+            let old_cells = ctx.regions[idx].cells.clone();
+            for &p in &old_cells {
+                ctx.tiles.set(p, Tile::Wall);
+            }
+            for &p in &cave {
+                ctx.tiles.set(p, Tile::Floor);
+            }
+
+            // Tighten bounds to the cave and replace the region's cells.
+            let region = &mut ctx.regions[idx];
+            region.bounds = bounding_rect(&cave);
+            region.cells = cave;
+        }
+    }
+}
+
+/// Finds the largest 4-connected component of floor cells (`!wall`) in a `w`×`h`
+/// grid, returned as the set of flat indices, or [`None`] if there is no floor.
+fn largest_floor_component(wall: &[bool], w: usize, h: usize) -> Option<Vec<usize>> {
+    let mut seen = vec![false; w * h];
+    let mut best: Option<Vec<usize>> = None;
+    for start in 0..w * h {
+        if wall[start] || seen[start] {
+            continue;
+        }
+        // BFS this component.
+        let mut comp = Vec::new();
+        let mut stack = vec![start];
+        seen[start] = true;
+        while let Some(i) = stack.pop() {
+            comp.push(i);
+            let (x, y) = (i % w, i / w);
+            let mut push = |nx: usize, ny: usize| {
+                let n = ny * w + nx;
+                if !wall[n] && !seen[n] {
+                    seen[n] = true;
+                    stack.push(n);
+                }
+            };
+            if x + 1 < w {
+                push(x + 1, y);
+            }
+            if x > 0 {
+                push(x - 1, y);
+            }
+            if y + 1 < h {
+                push(x, y + 1);
+            }
+            if y > 0 {
+                push(x, y - 1);
+            }
+        }
+        if best.as_ref().map(|b| comp.len() > b.len()).unwrap_or(true) {
+            best = Some(comp);
+        }
+    }
+    best
+}
+
+/// The axis-aligned bounding box enclosing every point in `cells` (empty rect for
+/// an empty slice).
+fn bounding_rect(cells: &[Point]) -> Rect {
+    let Some(&first) = cells.first() else {
+        return Rect::new(0, 0, 0, 0);
+    };
+    let (mut min_x, mut min_y, mut max_x, mut max_y) = (first.x, first.y, first.x, first.y);
+    for &p in &cells[1..] {
+        min_x = min_x.min(p.x);
+        min_y = min_y.min(p.y);
+        max_x = max_x.max(p.x);
+        max_y = max_y.max(p.y);
+    }
+    Rect::new(min_x, min_y, max_x - min_x + 1, max_y - min_y + 1)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::blackboard::Blackboard;
+    use crate::grid::Grid;
+    use crate::region::{ConnGraph, RegionId};
+    use std::collections::BTreeSet;
+
+    fn ctx_with_room(w: u32, h: u32, room: Rect) -> GenContext {
+        let mut ctx = GenContext {
+            tiles: Grid::new(w, h, Tile::Wall),
+            regions: Vec::new(),
+            graph: ConnGraph::new(),
+            blackboard: Blackboard::new(),
+        };
+        let cells: Vec<Point> = room.iter().collect();
+        for &p in &cells {
+            ctx.tiles.set(p, Tile::Floor);
+        }
+        ctx.add_region(RegionKind::Room, room, cells);
+        ctx
+    }
+
+    fn run(ctx: &mut GenContext, cfg: CaveConfig, seed: u64) {
+        let mut rng = Rng::from_seed(seed).fork("cave_shaper#0");
+        CaveShaper::new(cfg).apply(ctx, &mut rng);
+    }
+
+    #[test]
+    fn name_is_cave_shaper() {
+        assert_eq!(CaveShaper::new(CaveConfig::default()).name(), "cave_shaper");
+    }
+
+    /// A guaranteed cave in a large room: it is non-empty, smaller than the full
+    /// rect (organic), 4-connected, every cell is Floor, every cell stays inside
+    /// the original room rect, and the region's cells/bounds match the cave.
+    #[test]
+    fn cave_is_connected_floor_within_room() {
+        let room = Rect::new(0, 0, 22, 18);
+        let mut ctx = ctx_with_room(22, 18, room);
+        run(&mut ctx, CaveConfig { cave_chance: 1.0, min_room: 7, fill: 0.45, steps: 4 }, 0x1234);
+
+        let r = &ctx.regions[0];
+        assert!(!r.cells.is_empty(), "cave should carve cells");
+        assert!((r.cells.len() as i32) < room.w * room.h, "a cave is not the full rect");
+
+        let set: BTreeSet<(i32, i32)> = r.cells.iter().map(|p| (p.x, p.y)).collect();
+        for &p in &r.cells {
+            assert!(room.contains(p), "cave cell {p:?} escaped the room rect");
+            assert_eq!(ctx.tiles.get(p), Some(&Tile::Floor), "cave cell {p:?} not Floor");
+            assert!(r.bounds.contains(p), "cave cell {p:?} escaped bounds {:?}", r.bounds);
+        }
+
+        // 4-connected: flood from the first cell reaches all of them.
+        let mut seen = BTreeSet::new();
+        let mut stack = vec![(r.cells[0].x, r.cells[0].y)];
+        while let Some((x, y)) = stack.pop() {
+            if !set.contains(&(x, y)) || !seen.insert((x, y)) {
+                continue;
+            }
+            stack.extend([(x + 1, y), (x - 1, y), (x, y + 1), (x, y - 1)]);
+        }
+        assert_eq!(seen.len(), r.cells.len(), "cave must be one 4-connected blob");
+    }
+
+    /// A small room never becomes a cave; `cave_chance` 0.0 never caves anything.
+    #[test]
+    fn small_rooms_and_zero_chance_stay_clean() {
+        let small = Rect::new(0, 0, 5, 5);
+        let mut ctx = ctx_with_room(5, 5, small);
+        let before = ctx.regions[0].cells.clone();
+        run(&mut ctx, CaveConfig { cave_chance: 1.0, min_room: 7, fill: 0.45, steps: 4 }, 1);
+        assert_eq!(ctx.regions[0].cells, before, "small room must stay a clean rect");
+
+        let big = Rect::new(0, 0, 20, 16);
+        let mut ctx2 = ctx_with_room(20, 16, big);
+        let before2 = ctx2.regions[0].cells.clone();
+        run(&mut ctx2, CaveConfig { cave_chance: 0.0, min_room: 7, fill: 0.45, steps: 4 }, 1);
+        assert_eq!(ctx2.regions[0].cells, before2, "cave_chance 0.0 changes nothing");
+    }
+
+    /// Same seed reproduces an identical cave (tiles + region).
+    #[test]
+    fn caves_are_deterministic() {
+        let room = Rect::new(0, 0, 20, 16);
+        let cfg = CaveConfig { cave_chance: 1.0, min_room: 7, fill: 0.45, steps: 4 };
+        let mut a = ctx_with_room(20, 16, room);
+        let mut b = ctx_with_room(20, 16, room);
+        run(&mut a, cfg, 0x5EED);
+        run(&mut b, cfg, 0x5EED);
+        assert_eq!(a.tiles, b.tiles);
+        assert_eq!(a.regions[0].cells, b.regions[0].cells);
+        assert_eq!(a.regions[0].bounds, b.regions[0].bounds);
+    }
+}
diff --git a/reikhelm-core/src/passes/corridor.rs b/reikhelm-core/src/passes/corridor.rs
index 26e0de7..0093839 100644
--- a/reikhelm-core/src/passes/corridor.rs
+++ b/reikhelm-core/src/passes/corridor.rs
@@ -121,15 +121,31 @@ impl Pass for CorridorCarver {
     }
 }
 
-/// Returns the center of the `Room` region with id `index`, or [`None`] if the
-/// id is out of range or names a non-Room region.
+/// Returns a corridor anchor for the `Room` region with id `index`, or [`None`]
+/// if the id is out of range or names a non-Room region.
 ///
-/// Endpoint ids come from `MstConnect`, which only ever links real rooms, so in
-/// the assembled recipe this always resolves; the guard simply keeps the pass
-/// total against a hand-built or malformed graph.
+/// The anchor is the room's carved cell **nearest its bounding-box center**. For
+/// a convex, centered room (rectangle, octagon, ellipse, plus) the center cell is
+/// itself carved, so this is exactly `bounds.center()` — i.e. output-preserving.
+/// For an irregular room whose AABB center may fall in rock (a cave blob), it
+/// snaps to the nearest actual floor cell, so the corridor always meets carved
+/// floor and the room cannot be left unreachable.
+///
+/// Endpoint ids come from `MstConnect`, which only ever links real rooms (with
+/// non-empty `cells`), so in the assembled recipe this always resolves to a real
+/// cell; the guards keep the pass total against a hand-built or malformed graph.
 fn room_center(ctx: &GenContext, index: crate::region::RegionId) -> Option<Point> {
     let region = ctx.regions.get(index.0)?;
-    (region.kind == RegionKind::Room).then(|| region.bounds.center())
+    if region.kind != RegionKind::Room {
+        return None;
+    }
+    let target = region.bounds.center();
+    region
+        .cells
+        .iter()
+        .copied()
+        .min_by_key(|p| (p.x - target.x).pow(2) + (p.y - target.y).pow(2))
+        .or(Some(target))
 }
 
 /// Carves [`Floor`](Tile::Floor) at `p` (a bounds-safe no-op if `p` is off-grid)
diff --git a/reikhelm-core/src/passes/mod.rs b/reikhelm-core/src/passes/mod.rs
index cf3afbf..2ac1b1f 100644
--- a/reikhelm-core/src/passes/mod.rs
+++ b/reikhelm-core/src/passes/mod.rs
@@ -29,6 +29,9 @@ pub use bsp::{BspConfig, BspPartition};
 pub mod room;
 pub use room::{RoomCarver, RoomConfig, ShapeWeights};
 
+pub mod cave;
+pub use cave::{CaveConfig, CaveShaper};
+
 pub mod connect;
 pub use connect::{ConnectConfig, MstConnect};
 
diff --git a/reikhelm-core/src/recipes/dungeon.rs b/reikhelm-core/src/recipes/dungeon.rs
index f7221b2..def32c7 100644
--- a/reikhelm-core/src/recipes/dungeon.rs
+++ b/reikhelm-core/src/recipes/dungeon.rs
@@ -6,13 +6,15 @@
 //! order:
 //!
 //! ```text
-//! BspPartition → RoomCarver → MstConnect → CorridorCarver → DoorPlacer
-//!   → PoolDecorator → PillarPlacer
+//! BspPartition → RoomCarver → CaveShaper → MstConnect → CorridorCarver
+//!   → DoorPlacer → PoolDecorator → PillarPlacer
 //! ```
 //!
 //! 1. [`BspPartition`] cuts the canvas into leaf rectangles (one placeholder
 //!    Room region per leaf).
-//! 2. [`RoomCarver`] carves an actual room inside each leaf.
+//! 2. [`RoomCarver`] carves an actual room inside each leaf, after which
+//!    [`CaveShaper`] re-sculpts some rooms into organic cellular-automata caves
+//!    (each a single connected blob; the rest stay clean polygons).
 //! 3. [`MstConnect`] plans which rooms link up (a minimum spanning tree, plus
 //!    optional loop edges).
 //! 4. [`CorridorCarver`] carves an L-shaped floor corridor per planned edge.
@@ -43,8 +45,9 @@ use serde::{Deserialize, Serialize};
 
 use crate::pass::Pipeline;
 use crate::passes::{
-    BspConfig, BspPartition, ConnectConfig, CorridorCarver, DoorConfig, DoorPlacer, MstConnect,
-    PillarConfig, PillarPlacer, PoolConfig, PoolDecorator, RoomCarver, RoomConfig, ShapeWeights,
+    BspConfig, BspPartition, CaveConfig, CaveShaper, ConnectConfig, CorridorCarver, DoorConfig,
+    DoorPlacer, MstConnect, PillarConfig, PillarPlacer, PoolConfig, PoolDecorator, RoomCarver,
+    RoomConfig, ShapeWeights,
 };
 
 /// Configuration for the [`dungeon`] recipe.
@@ -63,6 +66,8 @@ pub struct DungeonConfig {
     pub bsp: BspConfig,
     /// How a room is carved inside each leaf.
     pub rooms: RoomConfig,
+    /// How some rooms are re-sculpted into organic cellular-automata caves.
+    pub caves: CaveConfig,
     /// How rooms are linked into a connectivity graph.
     pub connect: ConnectConfig,
     /// How room↔corridor pierce points are marked as doors.
@@ -101,6 +106,7 @@ impl Default for DungeonConfig {
                 margin: 1,
                 shapes: ShapeWeights::varied(),
             },
+            caves: CaveConfig::default(),
             connect: ConnectConfig {
                 extra_edge_ratio: 0.30,
             },
@@ -215,6 +221,7 @@ pub fn dungeon(cfg: DungeonConfig) -> Result<Pipeline, ConfigError> {
     let pipeline = Pipeline::new(cfg.width, cfg.height)
         .then(BspPartition::new(cfg.bsp))
         .then(RoomCarver::new(cfg.rooms))
+        .then(CaveShaper::new(cfg.caves))
         .then(MstConnect::new(cfg.connect))
         .then(CorridorCarver::new())
         .then(DoorPlacer::new(cfg.doors))
@@ -233,9 +240,10 @@ mod tests {
     use std::collections::{BTreeSet, VecDeque};
 
     /// The pass names in pipeline order, used to check snapshot labels.
-    const PASS_NAMES: [&str; 7] = [
+    const PASS_NAMES: [&str; 8] = [
         "bsp_partition",
         "room_carver",
+        "cave_shaper",
         "mst_connect",
         "corridor_carver",
         "door_placer",
@@ -306,7 +314,7 @@ mod tests {
         let (snapped, snapshots) = dungeon(cfg).unwrap().run_with_snapshots(7);
 
         assert_eq!(plain, snapped, "snapshotting must not change the map");
-        assert_eq!(snapshots.len(), 7, "one snapshot per pass (seven passes)");
+        assert_eq!(snapshots.len(), 8, "one snapshot per pass (eight passes)");
 
         let labels: Vec<&str> = snapshots.iter().map(|s| s.label.as_str()).collect();
         assert_eq!(